I think that you are right. An English teacher working privates, running around town 6 days a week for 12-14 hours is making 4.0-5.0 million; I've known more than a few teachers who fall into that category.
Anyone making more than that is teaching at a hogwon which he/ she has gone in on, or owns out right.
The reality is that the average salary of a teacher working at a hogwon, or university is 2.0-2.2 million, and if you include the apartment it pushes it to the 3.0 million. OK, some teachers under contract are also doing privates, so they might fall into the high 3.0 million, mid 4.0 million.
So,unless some teachers are running all over town, with no social life, or own, or partly a hogwon, I do not see any teacher making anything over 4.0 million.

I'm working on a DVD teaching series I plan to market through a Korean friend (and possibly hire some Koreans to sell on the subway). Total cost of production to me, including marketing costs, is 2,000 won per DVD. Sale price on subway will be 10,000 won (or a little less if sales don't go).

If I sold just 10 per day at 10,000 won, I'd net 80,000 a day. Take that times five days a week, times four weeks a month, and that's another 1.6 million a month.

Add that to the 4 million + I make currently teaching, and it only goes up from there.

How about foreigners teaching in universities? I guess they make about 5 million a month?

Pohang University of Science and Technology, which has the highest annual salaries at around 67 million won ($52,000). The universities that topped the list after Pohang were Yeungnam University (62.7 million won), Sungkyunkwan University (61.7 million won), Korea University (59.5 million won), Hannam University (58.3 million won) and Hoseo University (56.4 million won). The lowest-ranking school paid an average annual salary of 28 million won. The annual salary of first-year professors is 43 million won at Sungkyunkwan.

I think there may be 2 types of key money...I've heard of something like that, but in my case before, key money was 5 mil, but the rent was 700,000...must be the other kind where it's not a percentage of property value.

High paying companies offer W34 million annually (W2.83 million monthly) for new employees

New employees can expect to receive an annual average salary of
26.21 million won according to a survey of 120 local large corporations.

Annual salary for entry-level employees (new employees)
1. Halla Climate Control Corp., automobile air condition maker: 34 million won
2. Hana Bank: 34 million won
3. SK Telecom: 33 million won
4. KT Corp.: 32 million won
5. Woori Bank: 32 million won
6. Industrial Bank of Korea: 32 million won
7. Nine other companies: 30.52 million won
8. Shipbuilding and heavy industry sector: 29.5 million won
9. Automobile and aviation sector: 29 million won
10. Petrochemicals: 27.10 million won
11. Pharmaceutical: 26.70 million won
12. Construction: 25.25 million won
13. Steel and machinery industry: 24.83 million won
14. Food and beverage: 23.43 million won
15. Retail business: 23.20 million won
16. Electronics: 23 million won
http://www.koreaherald.co.kr/SITE/data/html_dir/2004/01/07/200401070037.asp
The starting annual salary for college graduates is 26 million won.
http://times.hankooki.com/lpage/biz/200312/kt2003123017152911910.htm

Advanced Full Payment for Entire Lease Period
This is the most common rental payment method in foreign communities in Seoul. For example, a 2-year lease agreement at $5,000/month would require $120,000 up front payment. No money is returned to the tenant at the end of the lease term.

Key Money Deposit Plus Monthly Payments (wolsei in Korean)
The key money is a security deposit in the amount of 10 to 20 times the monthly payment. The larger the key money deposit, the lower the monthly payment. Usually 10% of the key money is paid when the contract is signed. And the key money balance plus one month's rental is due when the tenant moves in. The key money deposit is returned to the tenant when the lease period expires.

Key Money Deposit Only (jeonsei in Korean)
Key money is required in the amount of approximately 40% to 60% of the property value to be held as security for the entire duration of the lease. The entire key money amount will be returned to the tenant upon expiration of the lease. Usually 10% of the total key money is due when the contract is signed. The balance is due when the tenant moves into the premises. The tenant is responsible for all maintenance and utilities unless otherwise stated in the lease agreement. The key money will not be returned prior to the expiration of the lease agreement until a new tenant signs a new lease agreement replacing the current one.

Be careful renting in KoreaFor Housing Rentals, Foreigners Easy Victims
Foreign residents in Seoul are preferred over any Korean tenant by their landlords because they are paying several times as much as what Koreans are paying for their rent, realtors and industry sources say. For a 25-pyong (one pyong equals 3.3 square meters) apartment in Seoul, foreign residents are paying as much as two million won ($1,600) in monthly rent while the same apartment may go for only 500,000 won for Koreans.
Korea Times
http://times.hankooki.com/lpage/nation/200308/kt2003082818233111970.htm

There is one way to find out if this is true. Tell a Korean friend the type of flat you want. Your friend then goes to the real estate agency in the area you want and acts as someone looking for a flat. Your friend will be told what the key money/ rent is and gets back to you.
Now, if information in hand you go to the same agency and see if the you are told the same thing.
Another way is to just look at the ads on the windows of agencies in the area you want to live in, and you will get an idea of the rents.
Having said all that, I am still of the mind set that most owners are honest and that a few bad apples make the rest of them look bad; I am looking for a place of my own and, having done my research, have not run into anybody who was trying to rip me off.

On another subject. Real Reality, with all the cut-and-pastes you do about how bad Korea is, and the dishonesty of Koreans, why are you still here?